LLDPE-based thermoshrinkable films

Details LLDPE-based thermoshrinkable films LLDPE-based thermoshrinkable films

1996-04-18

2004-02-10

Wilson, D. R. (Department: 1713)

Stock material or miscellaneous articles

Hollow or container type article

Shrinkable or shrunk

C428S035100, C428S910000, C428S516000, C525S240000

Reexamination Certificate

active

06689436

ABSTRACT:

This invention relates to heat-shrinkable films based on polyolefin compositions comprising linear copolymers of ethylene with &agr;-olefins having from 3 to 12 carbon atoms (LLDPE) and particular propylene copolymers, the said polyolefin compositions having particular mechanical properties.
The use of LLDPE in the production of single- or multi-layer heat-shrinkable films is widely known in the art. The said films are usually produced by extrusion of the polymer, obtaining a primary film which is then oriented by stretching under temperature conditions such that there is molecular orientation without there being any problems of tearing and breakage. The polymer used in the production of the film must therefore be plastic enough to be stretched, within the temperature range in which the orientation process takes place, and at the same time strong enough to withstand the stretching force. In addition, the temperature range within which these two properties co-exist must be as wide as possible, as narrow temperature ranges would make it difficult to manage the stretching and orientation process. The orientation can either be in a single direction or in two directions, usually perpendicular to each other (biaxial orientation). Biaxial orientation is generally carried out using, for example, the twin-bubble or tenter frame method. Once it has been oriented, the film has the ability—when subjected to a certain temperature—to shrink or to exert a shrinkage force when actual shrinkage is prevented, thus adhering perfectly to the object to be packaged.
In the case of single-layer films based on LLDPE, or in multi-layer films in which the various layers are made up essentially of LLDPE, it is usually necessary to cross-link the film, at least partially, before subjecting it to the orientation process, so as to give it adequate mechanical properties to withstand the stretching force. The cross-linking is usually effected by irradiating the film. As an alternative or in addition to cross-linking, LLDPE is blended with other polymers, such as ethylene/vinyl acetate (EVA) or VLDPE/ULDPE copolymers, to obtain films which have adequate mechanical properties and are easy to work. These techniques affect the economics of the production process and thus the final cost of the film.
Non-cross-linked films based exclusively on LLDPE can only be produced when LLDPE is a copolymer of ethylene with a higher &agr;-olefin, for example 1-octene. With other olefins, for example 1-butene or 1-hexene, the temperature needed for orientation is so close to the melting point of the polymer that in practice it is not possible to stretch the film, even at low speed, without there being breakages in the film itself. U.S. Pat. No. 4,597,920 describes a process for the production of heat-shrinkable films in which the polymer used is a copolymer of ethylene with an &agr;-olefin having 8-18 carbon atoms. The copolymer is characterized by two different melting points which allow the production of films on a commercial basis without cross-linking the polymer. The working range, however, is restricted; it can be seen in fact from the examples that the film can only be stretched with good results at temperatures between 115 and 120° C. In addition, LLDPEs modified with 1-octene or higher &agr;-olefins have significantly higher costs than conventional LLDPEs modified with 1-butene.
Patent application EP-A-434322 describes the use of blends of ethylene/1-hexene copolymers or ethylene/1-octene copolymers with significant quantities of LDPE. According to the description in the patent, the addition of LDPE to LLDPE modified with 1-hexene (which cannot be used as such in the production of heat-shrinkable films) makes it possible to use the mixture which is obtained in the production of heat-shrinkable films; the mixture of LDPE with LLDPE modified with 1-octene is claimed, however, to increase the extrusion rate and thus the productivity of the process. The working range remains very restricted, however.
Patent application WO 94/21726 describes single-layer bioriented heat-shrinkable films obtained from polymer compositions comprising an ethylene/&agr;-olefin copolymer, a polymeric alloy (made up of a heterophasic composition in which an amorphous ethylene/propylene copolymer is dispersed in a homopolymeric propylene matrix) and/or a random copolymer of propylene with ethylene, and possibly a crystallization inhibitor. According to what is reported in the said patent application, the compositions described allow good management of the blowing and orientation phase in twin-bubble processes. The preferred ethylene/a-olefin copolymer is an LLDPE modified with 1-octene; no examples are reported in which LLDPE is modified with 1-butene or 1-hexene.
It has now been found that by using particular polymer compositions based on LLDPE it is possible to produce heat-shrinkable films having very good mechanical properties (in some cases superior to those of cross-linked films) without using cross-linking treatments and without encountering any problems during the orientation process. Surprisingly, it has in fact been seen that polyolefin compositions comprising LLDPE and particular propylene copolymers can be used in the orientation processes without problems of tearing or breakages even when LLDPE is made up of an ethylene/1-butene copolymer.
The films of this invention are also characterized by a good set of optical and mechanical properties, in particular high tear resistance, which makes them particularly suited to use on high-speed packaging lines.
The heat-shrinkable films of this invention comprise a polyolefin composition containing: (i) from 80 to 100 parts by weight of a copolymer of ethylene (LLDPE) with one or more CH
2
═CHR &agr;-olefins, where R is a hydrocarbon radical having 1-10 carbon atoms, the said copolymer containing up to 20 mol % of CH
2
═CHR &agr;-olefin and having a density between 0.88 and 0.945 g/cm
3
; and (ii) from 5 to 30 parts by weight of a copolymer of propylene with one or more CH
2
═CHR
1
&agr;-olefins, where R
1
is a hydrocarbon radical having from 2 to 10 carbon atoms, and possibly with ethylene, the said copolymer containing from 60 to 98% by weight of units derived from propylene, from 2 to 40% by weight of units derived from the CH
2
═CHR
1
&agr;-olefin, and from 0 to 10% by weight of units derived from ethylene, and having a xylene-insoluble fraction greater than 70%.
Copolymer (i) is preferably present in amounts between 90 and 100 parts by weight and has a density preferably between 0.89 and 0.94 g/cm
3
. More preferably these values are between 0.90 and 0.935.
The Melt Index (determined by the ASTM D-1238 method, condition E) of copolymer (i) has values which are usually between 0.1 and 10 g/10 minutes, preferably between 0.2 and 5 g/10 minutes, more preferably between 0.2 and 3 g/10 minutes.
The CH
2
═CHR &agr;-olefin can for example be chosen from propylene, 1-butene, 1-hexene, 1-octene and 4-methyl-1-pentene; 1-butene or 1-hexene is preferably used. In the production of component (i) the CH
2
═CHR &agr;-olefins can also be used mixed.
Copolymer (i) is produced by copolymerization of ethylene with a CH
2
═CHR &agr;-olefin, in the presence of a catalyst of the Ziegler-Natta type obtained from the reaction of an organometallic compound of a metal in Groups II and III of the Periodic System with a catalytic component comprising a compound of a transition metal belonging to Groups IV, V or VI of the Periodic System. The transition metal compound is preferably supported on a solid support comprising a magnesium halide in an active form. Examples of catalysts that can be used in the production of copolymer (a) are described in U.S. Pat. No. 4,218,339 and U.S. Pat. No. 4,472,520, the description from which is included here for reference purposes. The catalysts can also be produced by the methods described in the patents U.S. Pat. Nos. 4,748,221 and 4,803,251.
Other examples of catalysts are described in the patent applications EP-A-395083, EP-A-553805 and EP-A

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